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Abstract

The Senepol cattle has a mutation in the myostatin gen (MSTN) which codifies for a high degree of skeletal muscle hypertrophy. However, thermoregulation, production, and reproduction related mechanisms need to be determined for these animals under heat stress. Thus, the effect of environment temperature (AT) was determined on the vaginal (VT), rectal (RT) and scrotal (ST) temperatures, and seminal characteristics of twenty cows (n= 20) and bulls (n=20) phenotypically and molecularly classified with intermediate muscle hypertrophy (HM) or normally muscled (HN). In the cows, the VT were collected every 5 minutes for a week, and calving interval (IBC), maternal calving ease (MCE), pregnancy rate (PR), calving rate (CR), maternal weight at calving (MWC), maternal weaning weight (MWW), and calf weight at birth (CWB), and at weaning (CWW) were evaluated. Bulls were collected during the months of August to September (hot season) and from January to February (cool season), initially to activate the spermatogenesis process and 60 days later. The air temperature (AT) was collected in both studies. The VT, RT, ST, productive, reproductive and seminal qualities data were analyzed in SAS by the GLIMMIX procedure. Also, molecular breeding values (MBV) for different traits of interest were evaluated in a population of 86 bulls and 53 cows according to a high (A), low (B), and intermediate (I) scale. Differences in VT between HM and HN cows were not observed (P>0.05), for hair type (TP; P>0.05). However, during the hot season, the HM bulls presented RT values 0.15°C higher than the HN ones (P=0.0232). Also during the hot season, the HM bulls presented higher sperm cells quantities with proximal cytoplasmic droplet (PCD) (26.62±2.81 vs 13.87±2.65; P= 0.0517) and less sperm cells number (CE/ml; 460.04x106 vs 887.32x106 sperm cells/mL; P=0.0098) than the HN group. The HN bulls presented greater scrotal circumference (SC) than the HM (36.12±0.31 vs 37.72±0.29 cm; P=0.005). In the group of cows productive and reproductive traits, differences were not observed (P>0.05). However, the MBV for the traits considered were only influenced by sex (P<0.05). According to the obtained results, under the thermally stressful environmental conditions of Puerto Rico, the HM heterozygous Senepol cows performance is not negatively affected. However, the evaluated bulls with this phenotype showed inferior performance than their normally muscled counterparts.